1992 Dodge Stealth Tachometer Signal

My '92 Dodge stealth has 140000 miles on it. Recently I have been having problems with my tachometer. Once the car is up to temperature, the tach needle will bounce around anywhere between 0 and what I assume the actual engine speed is. If the engine is up to temperature and I am on the highway and I drive at a constant speed, sometimes the tach needle will just stay at 0. When it bounces or stays at 0, the engine runs perfectly fine and does not correspond to what the tach read out is. Whats going on? Faulty sensor? Bad tach? ECM? I have no clue.